The term chromatography embraces a family of closely related separation methods based on the contacting of two mutually immiscible phases, wherein one phase is stationary and the other phase is mobile. One area wherein chromatography is of great interest is in the biotechnological field, such as for large-scale economic production of drugs and diagnostics. Generally, proteins are produced by cell culture, either intracellularly or by secretion into the surrounding medium. Since the cell lines used are living organisms, they must be fed with a complex growth medium containing sugars, amino acids, growth factors, etc. Separation of the desired protein from the mixture of compounds fed to the cells and from other cellular components to a sufficient purity, e.g. for use as a human therapeutic, poses a formidable challenge.
In such separation, in a first step, cells and/or cell debris is usually removed by filtration. Once a clarified solution containing the protein of interest has been obtained, its separation from the other components of the solution is often performed using a combination of different chromatography steps, often based on different separation principles. Thus, such steps separate proteins from mixtures on the basis of charge, degree of hydrophobicity, affinity properties, size etc. Several different chromatography matrices, such as matrices for ion exchange, hydrophobic interaction chromatography (HIC), reverse phase chromatography (RPC), affinity chromatography and immobilized metal affinity chromatography (IMAC), are available for each of these techniques, allowing tailoring of the purification scheme to the particular protein involved. An illustrative protein, which is of steadily growing interest in the medical field, is immunoglobulin proteins, also known as antibodies, such as immunoglobulin G (IgG).
As in all process technology, an important aim is to keep the production costs low. Consequently, improved chromatographic techniques are frequently presented, and the matrices are when possible reused. However, since each use of a chromatography matrix will leave certain traces of the operation just performed, many different cleaning protocols are available for cleaning and/or restoring the matrix into its original form. Commonly known materials that need to be removed are e.g. non-eluted proteins and protein aggregates as well as potentially hazardous materials, such as virus, endotoxins etc, which may originate from the cell culture. The most commonly used cleaning is a simple wash with buffer. For a more efficient cleaning of the matrix, treatments with acid and/or base are frequently used. In order to even more efficiently restore the matrix, an alkaline protocol known as Cleaning In Place (CIP) is commonly used. The standard CIP involves treatment of the matrix with 1M NaOH, pH 14. Such harsh treatment will efficiently remove undesired fouling of the above-discussed kind, but may in addition impair some chromatography matrix materials. For example, many affinity matrices, wherein the ligands are proteins or protein-based, cannot withstand standard CIP, at least not while maintaining their original properties. It is known that structural modification, such as deamidation and cleavage of the peptide backbone, of asparagine and glutamine residues in alkaline conditions is the main reason for loss of activity upon treatment of protein in alkaline solutions, and that asparagine is the most sensitive of the two. It is also known that the deamidation rate is highly specific and conformation dependent, and that the shortest deamidation half times in proteins have been associated with the sequences -asparagine-glycine- and -asparagine-serine. See e.g. Gülich, Linhult, Nygren, Uhlen and Hober (2000) Journal of Biotechnology 80, 169-178. Stability towards alkaline conditions can be engineered into a protein ligand.
Despite the documented alkaline sensitivity, protein A is widely used as a ligand in affinity chromatography matrices due to its ability to bind IgG without significantly affecting the affinity of immunoglobulin for antigen. As is well known, Protein A is a constituent of the cell wall of the bacterium Staphylococcus aureus. Such Staphylococcus protein, known as SpA, is composed of five domains, designated in order from the N-terminus as E, D, A, B, and C, which are able to bind antibodies at the Fc region, and a C-terminal region (or “X” region) that does not bind any antibodies. Jansson et al (Jansson, Uhlen and Nygren (1998) FEMS Immunology and Medical Microbiology 20, 69-78: “All individual domains of staphylococcal protein A show Fab binding”) have later shown that all the individual SpA domains also bind certain antibodies at the Fab region.
U.S. Pat. No. 5,151,350 (Repligen) relates to cloning and expression of the gene coding for a protein A and protein A-like material. The cloning of this gene with its nucleotide sequence characterization enabled in 1982 for the first time to obtain quantities of a protein A-like material and nucleotide sequence for cloning in various host-vector systems.
Since the production of protein A in a recombinant system was accomplished, further genetic manipulations thereof have been suggested. For example, U.S. Pat. No. 5,260,373 (Repligen) describes genetic manipulation of recombinant protein A in order to facilitate the attachment thereof to a support, and more specifically to the coupling thereof via arginine. Further, U.S. Pat. No. 6,399,750 (Pharmacia Biotech AB) describes another recombinant protein A ligand, which has been coupled to a support via cysteine.
However, in order to maintain selectivity and binding capacity, Protein A chromatography matrices of the above-discussed kind need to be cleaned under milder conditions than conventional CIP. In this context, it is understood that the cleaning is closely related to the lifetime of the chromatography matrix. For example, a sensitive matrix may be cleaned with standard CIP, if a reduced performance is acceptable. Thus, efforts have been made to provide chromatography matrices which present the outstanding properties, such as selectivity, of protein A, but which are more resistant to alkaline conditions used for CIP.
Thus, U.S. Pat. No. 6,831,161 (Uhlen et al) relates to methods of affinity separation using immobilized proteinaceous affinity ligands, wherein one or more asparagine (Asn) residues have been modified to increase alkaline stability. This patent also describes methods of making a stabilized combinatorial protein by modification of Asn residues within a protein molecule to increase stability of the protein in alkaline conditions, and randomization of a protein molecule to modify its binding characteristics, and combinatorial proteins wherein in a step separate from the randomization step, the stability of the protein in alkaline conditions has been increased by modifying one or more of its Asn residues.
Further, WO 03/080655 (Amersham Biosciences) relates to an immunoglobulin-binding protein, wherein at least one asparagine residue has been mutated to an amino acid other than glutamine or aspartic acid. According to this patent application, such more specific mutation confers an increased chemical stability at pH-values of up to about 13-14 compared to the parental molecule. The mutated protein can for example be derived from a protein capable of binding to other regions of the immunoglobulin molecule than the complementarily determining regions (CDR), such as protein A, and preferably from the B-domain of Staphylococcal protein A. The invention also relates to a matrix for affinity separation, which comprises the described mutated immunoglobulin-binding proteins as ligands.
Despite the above-described development towards more alkaline-stable protein A-based chromatography ligands, there is still a need in this field of improved ligands and chromatography matrices for highly specific isolation of antibodies, and of alternative wild type ligand constructions that allow easier manufacture.
One example of such an improved chromatography matrix is described in US 2006/0134805 (Berg et al), which relates to a separation matrix comprised of porous particles to which antibody-binding protein ligands have been immobilised. More specifically, the disclosed chromatography matrix has been optimised in terms of ligand density; gel phase distribution coefficient (Kay); and particle size to provide a matrix especially suitable for high capacity purification of antibodies. The ligands of the disclosed matrix may comprise antibody-binding protein such as Protein A, Protein G and/or Protein L.